The present invention relates to a device to measure the flowrate of a fluid flowing in a partially full line, such as a canal or open channel, which has (1) coils arranged above the partially full line and below the bottom of the partially full line to generate a magnetic field essentially vertical to the partially full line, (2) electrodes arranged on the opposite walls of the partially full line opposite to each other, (3) an excitation circuit for the coils, and (4) an evaluation circuit which processes the potential difference at the electrodes resulting from the magnetic field and the fluid flow and which produces a signal proportional to the flowrate of the fluid flowing through the partially full line.
A known device of this type, described in DE-OS 20 63 792, has coils with horizontal axes which are located on the sides of the walls of the partially full line and which, when energized with opposing polarities, generate a magnetic field which is essentially perpendicular to the middle of the partially full line. Such a device is costly because of the requirement for the coils to be located on the walls of the partially full line. Also, this device does not produce a magnetic field distribution which results in a strong and easily evaluated flow signal in the partially full line.